BACKGROUND: The prevalence of allergic diseases markedly varies according to the geographical region. OBJECTIVE: To determine the prevalence of asthma, allergic rhinitis and atopic dermatitis in children of six and seven years of age. METHODS: Cross-sectional study that included a sample of almost 15 thousand children in five cities of Mexico. To obtain the prevalence of allergic diseases and their symptoms, the parents of the children answered The International Study of Asthma and Allergies in Childhood questionnaire. 95% confidence intervals (CI) were estimated for proportions. RESULTS: Of the total sample, 7466 (52.5 %) were boys and 7463 (47.5%) girls. In general, the prevalence of asthma, exercise-induced asthma and current asthma was 6.1 % (95% CI = 5.7%-6.5%), 2.1 % (95% CI = 1.9%-2.3%) and 11.6 % (95% CI = 11.1% -12.1%), respectively. On the other hand, the prevalence of allergic rhinitis was 5.4 % (95% CI = 5.1%-5.8%), rhinitis with conjunctivitis, 19.2 % (95% CI = 18.6%-19.8%), and current rhinitis, 52.8% (95% CI = 52.0%-53.6%). The prevalence of atopic dermatitis and current dermatitis was 3.4 % (95% CI = 3.1%-3.7%) and 7.8% (95 % CI = 7.4%-8.2%), respectively. CONCLUSIONS: Our results indicate that the prevalence of allergic diseases in school children is similar to that observed in previous studies in Mexico.